The major goal of PA-97-060, Exploratory grants for multidisciplinary clinical studies of sarcopenia, is to facilitate the use of novel techniques and to develop new collaborations investigating changes in skeletal muscle morphology and metabolism that lead to physical disabilities and sarcopenia of old age. The age-associated reduction in skeletal muscle mass, defined as sarcopenia, muscle strength and functional capacity have been well described. Preliminary data suggest that loss of muscle mass alone does not explain all of the losses in strength and functional properties of skeletal muscle of older individuals, and these losses may be ascribed to losses in the amount of contractile protein in the muscle. The investigators propose a project to examine the relationship between physiological and structural properties of skeletal muscle taken from young and old individuals and their in vivo muscle size (from CT), strength, and power. The proposed investigation will examine single muscle fiber contractile propertied and the content of the contractile proteins myosin heavy chain and actin from the same biopsy. The older men and women recruited for this study will be over the age of 75 y and carefully screened for the absence of cardiovascular, cerebrovascular, neuromuscular, or arthritic disease. Healthy young men and women between the ages of 20 & 30 y will also be recruited. They will test the hypothesis that differences in muscle strength and power between young and old individuals result from differences in individual myofiber contractile properties and the amount of the contractile proteins myosin and actin within the individual muscle fibers.